1. Field of Invention
The present invention relates to a signal readout circuit, and more particularly to a signal readout circuit applied in a wide current-sensing range of amperometric chemical sensing.
2. Related Art
The earliest amperometric signal readout circuit was proposed by Turne, Harrison, and Baltes in 1987, and its major architecture is a two-electrode potentiostat. As the two-electrode sensing mode has a problem of concentration polarization, a three-electrode potentiostat was developed. However, the conventional operational mode is to convert an output signal—current into a voltage signal, in which a voltage range is limited by a power supply.
FIG. 1 is a schematic view of a typical three-electrode signal readout circuit. The signal readout circuit is formed by a first amplifier 11, a second amplifier 12, and a third amplifier 13. A first input terminal of the first amplifier 11 is connected to a second electrode RE. A second input terminal of the first amplifier 11 is connected to an output terminal. A second input terminal of the second amplifier 12 is connected to the output terminal through a second resistor R2. The second input terminal is also connected to a first electrode WE. A first input terminal of the second amplifier 12 is connected to a ground terminal. An output terminal of the second amplifier 12 is an output terminal of a sensed signal. A first input terminal of the third amplifier 13 is connected to the ground terminal, and a second input terminal of the third amplifier 13 is connected to the output terminal of the first amplifier 11 through a first resistor R1, and another voltage −Vox is input to the second input terminal through a third resistor R3. An output terminal of the third amplifier 13 is connected to a third electrode CE. A sensing current Isensor is generated at the first electrode WE at first. An output voltage Vo of the output terminal of the third amplifier 13 is Isensor×R2.
The three-electrode signal readout circuit shown in FIG. 1 uses too many operational amplifiers (OPAs). In addition, the circuit needs dual power supply and has a problem of excessive power consumption. Therefore, in recent years, many improved circuits of potentiostats were proposed. However, these circuits have disadvantages such as excessive consumption, large chip areas, small sensing ranges, and nonadjustable measuring ranges, and the like.
FIG. 2 is a schematic view of another three-electrode signal readout circuit, in which a first amplifier 21, a first transistor M1, a second transistor M2, a third transistor M3, a fourth transistor M4, a first transistor M1, a second transistor M2, a third transistor M3, and a fourth transistor M4 form a cascode current mirror.
A resistor Rwr, a resistor Rrc, and a capacitor Cs are equivalent resistors and capacitor of a glucose solution. The resistor Rwr is a Faraday resistor, and a resistance of the resistor Rwr will decrease with the increase of the sensing current. The resistor Rrc is solution impedance. The capacitor Cs is an electric double layer capacitor of a working electrode. The resistor Rrc and the capacitor Cs will not change with currents. A resistor RF and a capacitor CF form a frequency compensation element of the circuit. A voltage Vox is an input pin for setting a potential of a second electrode RE. The sensing current Isensor is generated at the first electrode WE at first, flows through the resistor Rwr and the resistor Rrc and then enters the third electrode CE, and copies an output current Io through the cascode current mirror of the first transistor M1, the second transistor M2, the third transistor M3, and the fourth transistor M4. A Rin1 is an equivalent input resistor viewed from a CE terminal of the third electrode (a drain of the third transistor M3). The resistance of Rin1 is as shown in the first equation, that is, about mega ohms. Therefore, the resistor has very large input impedance, and is not suitable for serving as a current input terminal.
                              Rin          ⁢                                          ⁢          1                ≅                              g                          m              ⁢                                                          ⁢              3                                ⁢                      r                          o              ⁢                                                          ⁢              3                                ⁢                      r                          o              ⁢                                                          ⁢              1                                                          (        I        )                                                      A            1                    ⁢                                    β              1                        ⁡                          (              s              )                                      ≅                                            A              1                        ⁡                          (              s              )                                ·                                    g                              m                ⁢                                                                  ⁢                1                                                    g                              m                ⁢                                                                  ⁢                3                                              ·                      R            sensor                                              (        II        )            
The loop gain is as shown in the second equation: when Rsensor (that is, Rwr) changes, a maximum value of the loop gain is also influenced, which has a negative impact on the stability.
The signal readout circuit of the amperometric sensor shown in FIG. 2 has advantages such as low power and a small chip area. However, as the equivalent input resistance (Rin1) of the readout circuit is too large, signal oscillation will occur when the sensing current is smaller than a microampere level.